The invention relates generally to radio-engineering and particularly, to test benches for radio-technical measurements and experiments with Global Navigation Satellite signals (GNSS) such as GPS, GLONASS, Galileo, Compass, QZSS.
Apparatuses of GNSS positioning and navigation may operate when in the line of sight there are any obstructions between a transmitting satellite antenna and a receiving antenna of the apparatus. Such obstacles include mountains, hills, buildings, bridges, tunnels, masts, electric lines, trees, bushes etc.
The obstacles can distort (weaken and alter) the original satellite signal.
When satellites move across the sky, the satellite signal deteriorates in tree tops and reflects many times from the bottom crown limit and the ground, thereby resulting in sharp and chaotic signal oscillations in phase and amplitude at the receiving antenna, as well as possible phase slips by 180 and over degrees.
Deteriorations in the original satellite signal cause malfunctions of positioning and navigation equipment. Such signal distortions at the output of the receiving antenna are equivalent to amplitude and phase distortions in the directional diagram (DD) of the receiving antenna. These oscillations distortions take the form of oscillations of the DD in terms of amplitude and phase versus satellite elevation. Amplitude and phase of oscillations are further described with the notions: amplitude and phase directional diagrams (DD) that show a dependence of amplitude F(θ) [dB] and phase Φ(θ) [degree] for received signals on the direction of signal arrival.
The pattern of distorting the original satellite signal depends on the shape and size of shading obstacles and their properties (conductivity, humidity and so on).
In addition, some properties of the shading obstacles may abruptly and unpredictably change. The surface of these obstacles can also change its humidity and conductivity due to rain or snow. Tree crowns and bushes can change their shape due to the wind, affecting obstacle shape and position relative to the receiving antenna of the apparatus.
A separate type of distortions is one caused by multi-reflections of the original satellite signal. A signal from a satellite can pass through a semi-transparent obstacle, then reflect from the bottom part of the obstacle (for example, from leaves of the lower tree offshoots) towards the ground and finally reflect from the ground and so on.
Such reflections can be geometrically very complex. Different “replicas” of the original satellite signal are received by a receiving antenna of a positioning and navigation unit. These replicas are distorted in phase and amplitude both relative to the original signal and to each other.
To compare and debug operation of positioning systems when obstacles are available/unavailable, one needs to artificially create such signal distortions.
These are test benches that are used in radio engineering for investigating and modeling operation of positioning units with/without distortions caused by signal reflections.
U.S. Pat. No. 7,650,261 discloses a multi-path generating apparatus simulating echo signals being repeated at different delays to model reflections from objects and a method of utilizing such and apparatus. But such a generator does not allow to model distortions specific for multi-path reflections from semi-transparent obstacles.
There is known a method of developing test benches in the form of a vertically-installed conducting semi-plane (a metal plate) (P. Ya. Ufimtsev, Theory of Edge Diffraction in Electromagnetics, Tech Science Press, Encino, Calif., 2003).
Such an installation provides full blockage in a shadow zone of the semi-plane, as well as amplitude and phase distortions of the signal at the antenna output when the satellite moves from the full antenna blockage area caused by the semi-plane to the line-of-sight area between antenna and satellite, in the so-called transition zone.
However this method has the following disadvantages.
It does not allow amplitude and phase oscillations of the signal at the antenna output when the satellite angularly travels over the whole upper hemi-sphere.
The vertical size of a metal plate is about 10 meters which makes assembling and usage of the test bench inconvenient.
In addition, interaction of satellite signals with the metal plate strongly depends on satellite orientations relative to the plate.
For example, if the receiving antenna is located within illuminated zone of said metal plate, reflections from the metal plate affect the antenna (the so-called upper multi-path). This multipath is partly filtered by the antenna based on polarization properties.
When the antenna is located in the shadow zone, an edge wave, excited by a metal plate edge, mainly contributes; and the signal of the edge wave quickly fades at setting the satellite in the shadow.
Therefore, the test bench in the form of the conducting semi-plane (metal plate) does not succeed in obtaining signal distortions shaped as sharp and multiple oscillations in the antenna DD in phase and amplitude which are relevant to multipath reflections from semi-transparent obstructions.
The same distortions, in particular, appear at locating the antenna under tree crowns/foliage.
The present invention aims development test radio engineering benches simulating characteristics of real obstructions causing multipath reflections and distortions of satellite signals.